Battery charger capable of independently charging electromagnetic cells

ABSTRACT

A current limited rechargeable battery charging apparatus is utilized to charge batteries comprising lithium type electrochemical cells. The charging apparatus of the present invention may be specifically utilized with rechargeable batteries comprising lithium type electrochemical cells utilized in a multiple cell series configuration. The lithium battery may be charge limited float charged such that the charging current does not exceed a predetermined current value. The charge current limiting may be provided without utilizing complex or expensive current control circuitry. The current limited rechargeable battery charging apparatus my be utilized as a part of a battery charging and utilization system implemented with portable electronic devices such that the electronic device may be utilized and the battery may be charged while simultaneously providing operational power and being overcharged protected. An external charging unit may be controlled from a battery pack containing the rechargeable battery and charge control circuitry.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Pat. No. 5,818,199filled Nov. 19, 1996, which claims priority pursuant to 35 U.S.C. Sec.119(e) to U.S. Provisional Application Ser. No. 60/006,966 (AttorneyDocket No. DN38137+), filed Nov. 20, 1995, which is hereby incorporatedherein by reference in its entirety. The present application alsoincorporates herein by reference in their entirety U.S. ProvisionalPatent Application Ser. No. 60/001,499 (Attorney Docket No. DN38134+),filed Jul. 18, 1995 and U.S. Application Ser. No. 08/687,007 (AttorneyDocket No. DN38134R1), filed Jul. 18, 1996.

TECHNICAL FIELD

The present invention relates generally to charging devices for lithiumtype electrochemical cells and specifically to a current limitedcharging apparatus for critical charging of lithium batteries or thelike.

BACKGROUND OF THE INVENTION

Demand for batteries with higher energy densities at lower costs isfueling the search for alternatives to traditional nickel-cadmium orlead-acid electrochemical cells. For example, nickel-metal hydrideelectrochemical cells have greater energy densities than nickel-cadmiumcells but are correspondingly more expensive. Zinc-air cells offergreater energy densities as well but cannot be rapidly charged and haveshorter operational life spans. Lithium-ion type cells are compact,light, offer high energy densities, have a high discharge voltage andexhibit stable discharge characteristics. Lithium-ion cells are wellsuited to applications requiring current draw for extended periods oftime and are relatively inexpensive.

Traditionally, lithium type batteries have not been widely utilized as amain power source in the portable electronics industry because of theinherent characteristics of lithium electrochemical cells that presentengineering difficulties. Effective and efficient charging of lithiumcells, especially when utilized in a series configuration, is oftendifficult to accomplish. Manufacturers of lithium type electrochemicalcells typically dictate stringent charging conditions to maximize thecharge and to prevent damage caused by overcharging. The maximum chargevoltage and current applied to the cells must be limited topredetermined levels.

Additionally, lithium electrochemical cells are susceptible toself-discharge due to the internal presence of hydrogen in theelectrochemical cells. For example, when a battery pack comprisesmultiple series connected lithium cells, the voltages of the cells aretypically mismatched due to the varying presence of internal hydrogen.This voltage mismatch causes the battery pack voltage to be lower thanits nominal value and results in rapid loss of electric charge. Duringcharging of the battery, the charging process may completely charge someof the cells and completely discharge other cells. The mismatched cellsultimately fail and thereby cause the instability and premature failureof the battery pack. Additionally, because some of the lithium cells arecompletely charged while others are discharged, the resulting totalseries voltage of the lithium cells is less than the nominal fullycharged voltage of the battery back thereby causing the battery pack tobe overcharged. Overcharging of the battery pack ultimately leads topremature failure and may possibly result in a fire or explosion of theelectrochemical cells.

It is for these mentioned reasons that lithium electrochemical cellshave not been widely utilized as the main power source in portablebattery powered electronic equipment. The charging process is complexand requires that the charging process is carefully monitored andcontrolled so that the charging voltage and current is strictly limited.Additionally, the lithium battery charging apparatus must account forany voltage mismatch between each of the individual cells. Thus, thebattery charger and battery pack typically requires complicatedconstruction and complex, expensive charging circuitry.

SUMMARY OF THE INVENTION

The present invention provides simple, effective, efficient, and lowcost charging of lithium-ion type electrochemical cells in a portablebattery powered electronic system. The charging current and voltage areefficiently limited and controlled within the stringent limits dictatedby lithium cell manufacturers. A cell balancing circuit may be utilizedwhich provides voltage equalization of the cell output voltages duringcharging in conjunction with the current and voltage limiting featuresof the present invention.

The charging apparatus of the present invention may be specificallyutilized with rechargeable batteries comprising lithium typeelectrochemical cells utilized in a multiple cell series configuration.The lithium battery may be charge limited float charged such that thecharging current does not exceed a predetermined current value. Thecharge current limiting may be provided without utilizing complex orexpensive current control circuitry. The current limited rechargeablebattery charging apparatus my be utilized as a part of a batterycharging and utilization system implemented with portable electronicdevices such that the electronic device may be utilized and the batterymay be charged while simultaneously providing operational power andbeing overcharged protected. An external charging unit may be controlledfrom a battery pack containing the rechargeable battery and chargecontrol circuitry.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous objects and advantages of the present invention may bebetter understood by those skilled in the art by reference to theaccompanying figures in which:

FIG. 1 is a schematic diagram of a current limited lithium batterycharging apparatus of the present invention;

FIG. 2 is a schematic diagram of the current limited lithium batterycharging apparatus of the present invention illustrating a preferredembodiment thereof, and

FIG. 3 is a schematic diagram of a current limited battery chargingsystem of the present invention for utilization in conjunction with aportable electronic device.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT

FIG. 1 illustrates schematically a current limited lithium batterycharging apparatus of the present invention. The problem which thepresent invention solves is effective and efficient charging of lithiumbatteries which comprise lithium type electrochemical cells especiallywhen utilized in a series configuration. Lithium batteries requirestringently controlled charging conditions including limiting themaximum charge voltage and current applied to the cells. The lithiumbattery system 10 typically includes a lithium battery 12 andaccompanying charge control electronics. The lithium battery 12 maycomprise two individual lithium type electrochemical cells V₁ and V₂electrically connected in series to achieve a battery output voltageV_(BATT) which is the sum of the output voltages of each individual cellV₁ and V₂.

During the charging cycle, the lithium battery system 10 receives adirect current input charge voltage V_(IN) which provides the electricalenergy to charge the lithium battery 12. The charge input voltage V_(IN)from typical supply sources may drift over a wide voltage range that istoo unregulated to properly and effectively charge the battery 12. In apreferred embodiment of the present invention, the cells V₁ and V₂ arevanadium-lithium cells having a nominal output voltage of 3.4 volts percell when fully charged requiring the charging voltage V_(IN) to be atleast 6.8 volts in order to fully charge the battery 12. The chargevoltage V_(IN) will typically be higher than 6.8 volts to compensate forany voltage drops from the voltage and current regulating circuits.

The charging input voltage V_(IN) is preferably amplitude regulated witha linear voltage regulator ("LINEAR REGULATOR") 14 to provide a voltageregulated charging voltage to the battery 12 with no voltage drift. Thevoltage regulator 14 preferably produces a 6.8 volt regulated voltageV_(REG) at the output 18 of the regulator 14 to charge thevanadium-lithium cells V₁ and V₂. To achieve a regulated output V_(REG)of 6.8 volts typically requires the voltage V_(IN) at the input of theregulator 16 to range from 8.0 volts to 9.6 volts. A reference signal 20proportional to the regulated output voltage V_(REG) typically may befed back to the regulator 14 via a voltage divider comprising resistorsR₁ and R₂.

The regulated charging signal V_(REG) preferably feeds through a chargecurrent limited ("CURRENT LIMITER") 22 the output of which 24 chargesthe battery 12 at positive battery terminal node 24. Preferably, thelithium type electrochemical cells are current limited float charged inthat the charging current I_(CHG) may not exceed 4.0 milliamperes. Ifthe charging current I_(CHG) were to exceed 4.0 milliamperes then thebatteries would be susceptible to being damaged from the chargingprocess. Thus, the linear voltage regulator 14 preferably provides aconstant, drift fee charging voltage V_(REG) to the lithium battery 12with the charging current I_(CHG) being limited by the current limiter22.

Lithium type electrochemical cells further require cell balancingtechniques when being charged to prevent any voltage mismatch on thecells. A voltage equalizer ("VOLTAGE EQUALIZER") 26 preferably connectsat the node 28 between the cells V₁ and V₂ and the charge input node 24and is utilized to balance the cell voltage during charging. Exemplaryvoltage equalizing circuits for multicell lithium battery applicationsare described in U.S. Provisional Patent Application Ser. No. 60/001,499(Attorney Docket No. DN38134+), filed Jul. 18, 1995 and U.S. PatentApplication Ser. No. 08/687,007 (Attorney Docket No. DN38134R1), filedJul. 18, 1996, both of which are incorporated herein in their entirety.

FIG. 2 illustrates a preferred current limited battery chargingapparatus of the present invention. The lithium battery system 10 of thepresent invention preferably utilizes vanadium-lithium electrochemicalcells producing an output voltage of 3.4 volts per cell when fullycharged and an output voltage of 3.0 volts per cell when discharged.Thus, the battery output voltage V_(BATT) ranges from 6.0 volts whendischarged to 6.8 volts when fully charged. The battery charging currentI_(CHG) must be limited to a maximum of 4.0 milliamperes. The battery 12will sink the most charging current when discharged and preferably willsink no current when fully charged. By analysis of the node voltages atnodes 18 and 24, the impedance of the current limiter 22, Z_(CL) (t),may be described as:

    Z.sub.CL (t)=[V.sub.REG (t)-V.sub.BATT (t)]/I.sub.CHG (t)

With the battery 12 fully discharged at t=0 seconds and the limitingconditions of a maximum charging current of 4.0 milliamperes, theimpedance of the current limiter 22 becomes:

    Z.sub.CL =[6.8 volts-6.0 volts]/4.0 milliamperes=200 ohms

Thus, a linear impedance, such as a current limiting resistor R_(L), isthe preferred implementation of the charge limiting function of thecurrent limiter 22. The current limiting resistor R_(L) preferably has avalue of 200 ohms when two vanadium-lithium cells are utilized in amutlicell series configuration. Since the linear regulator 14 alwaysproduces an output voltage of 6.8 volts at node 18, and because theoutput voltage V_(BATT) of the battery 12 is 6.0 volts when discharged,the charge current I_(CHG) will be limited to a maximum of 4.0milliamperes. Utilization of resistor R_(L) as a current limitereliminates the need for complex and expensive linear integratedcircuitry to perform the current limiting functions which is normallyrequired with the stringent and complex charging requirements of lithiumtype batteries.

FIG. 3 illustrates the lithium battery charging apparatus of the presentinvention utilized with a portable electronic device. The lithiumbattery 12 of the present invention is suitable for utilization inportable electronic devices such as hand-held portable data collectionterminals, for example. Hand-held portable data terminals are typicallyutilized in warehouse, product delivery and route accountingapplications wherein the terminal is utilized for several hours at atime before the battery is able to be recharged. Additionally, hand-helddata terminals typically utilize increasingly powerful integratedelectronic circuitry having greater power requirements as a result.Lithium type cells, once avoided by engineers because of technicallydifficult charging and discharging requirements, are now becomingembraced as higher power replacements to traditional nickel-cadmium andnickel-metal hydride batteries. Thus, the battery charging system may beimplemented with a hand-held portable data terminal system.

Battery charging apparatuses normally require bulky and heat generatingpower conditioning electronics. Because reduced size and increasedportability of the hand-held data terminal are important designconsiderations, bulky battery charging apparatuses are preferably notdesigned as a part of the terminal. The present invention utilizes abattery charging unit 30 external to the terminal 40. The charging unit30 preferably receives at its input 32 an alternating current (ac)voltage VAC such as typical household and office electrical power. Thecharging unit 30 preferably includes a power transforming and rectifyingcircuitry ("X_(F) ") 34 which transforms the ac power into a steppeddown direct current (dc) voltage suitable for powering the terminal 40and for recharging the battery 12. Further power conditioning circuitry("CVCC") 38 receives the dc voltage at its input 36 to maintain the dcvoltage at a constant voltage and constant current which are preferablyalways less than 8.4 volts and 1.0 amperes in a preferred embodiment ofthe present invention.

The charging unit 30 provides operational power to the terminal 40 atits power input 42. The dc power signal preferably passes through theterminal 40 to charge the battery 12, which is preferably utilized in aself-contained battery pack 52, via power input line 44. The batterypack 52 is designed to be utilized in conjunction with terminal 40 andpreferably includes the lithium battery 12 and operational circuitry ofthe present invention. The dc power signal is delivered to the batterypack 52 via input line 44 form the terminal 40 and passes through acharge control switch 50 which controls the flow of power into and outof the battery pack 52. In a preferred embodiment of the presentinvention the charge control switch is a p-channel field-effecttransistor.

When the charging switch 50 is closed the dc power signal from thecharging unit 30 may flow into the battery pack 52 and thereby chargethe battery 12. When the charging switch 50 is open, the chargingcontrol unit 30 is thereby prevented from delivering power to thebattery 12. A diode 48 connected in parallel with charging switch 50allows for only unidirectional flow of current when the charging switch50 is open. In this arrangement, the battery 12 may deliver power to theterminal 40 while power is prevented from being delivered to the battery12 to prevent overcharging. The thus described power control arrangementis especially applicable to protect the battery 12 from accidentalovercharging when the terminal 40 is being utilized and the battery 12is being charged simultaneously. The battery 12 may provide operationalpower to the terminal 40 during simultaneous charging while beingprotected from overcharging.

The operational functions of the battery pack 52 are preferablycontrolled with a microcontroller ("μC") 46 contained therein. Themicrocontroller 46 monitors and controls the flow of power into and outof the battery pack 52 and controls the charging of the battery 12. Themicrocontroller 46 measures the input voltage received by the batterypack 52 from line 44 through resistors R₅ and R₆. The microcontroller 46also monitors the output voltage of the battery 12 at node 24 throughresistors R₃ and R₄. Through proportional scaling of the voltagesthrough voltage dividers formed by resistors R₅ and R₆ and resistors R₃and R₄, the microcontroller 46 essentially charges battery 12 until theinput voltage at line 44 equals the output voltage of the battery 12 atnode 24.

The manner, routine and rate of charging of the battery 12 is preferablycontrolled by the microcontroller 46 via the switching on and off ofcharge control switch 50. With the preferred charge control arrangementof the present invention thus described, the microcontroller 46 of thebattery pack 52 controls the output of the external charging unit 30without requiring any charging control circuitry to be physicallylocated within the charging unit 30 and without requiring the chargingunit 30 to be located within the terminal 40 or within the battery pack52 and generate heat and occupy precious space. Further, no controlsignal is required to be passed from the battery pack 52 or the terminal40 to the charge unit 30 to control the charging function, furthereliminating the need for the utilization of control lines in addition tothe power supply line 44.

In view of the above detailed description of a preferred embodiment andmodifications thereof, various other modifications will now becomeapparent to those skilled in the art. The contemplation of the inventionbelow encompasses the disclosed embodiments and all reasonablemodifications and variations without departing from the spirit and scopeof the invention.

I claim:
 1. A battery charging and utilization system comprising:anelectronic device; a power unit, external to the electronic device, thatdelivers power to the electronic device when the power unit is coupledto the electronic device; a battery pack, removably insertible into theelectronic device, comprising charging circuitry and at least twoelectrochemical cells arranged in a series configuration to deliverpower to the electronic device when the electronic device is not coupledto the power unit; the charging circuitry including a current regulatorcoupled in series with the at least two electrical cells; and thecharging circuitry independently controlling charging of each of the atleast two electrochemical cells from power provided by the power unitwhen the electronic device is coupled to the power unit.
 2. The batterycharging and utilization system of claim 1 wherein the chargingcircuitry of the battery pack attempts to prevent overcharging of eitherof the at least two electrochemical cells while attempting to fullycharge both of the at least two electrochemical cells.
 3. The batterycharging and utilization system of claim 2 wherein the at least twoelectrochemical cells comprise lithium type cells.
 4. The batterycharging and utilization system of claim 2 wherein the chargingcircuitry comprises a voltage equalizer.
 5. The battery charging andutilization system of claim 2 wherein the charging circuitrysimultaneously charges the at least two electrochemical cells while inthe series configuration via access to a node formed between the atleast two electrochemical cells.
 6. The battery charging and utilizationsystem of claim 5 wherein the battery pack and power unit attempt toprovide continuous power delivery to the electronic device while eitherthe battery pack or the power unit couples or uncouples from theelectronic device.
 7. A battery system for an electronic devicecomprising:a power unit, external to the electronic device, thatdelivers power to the electronic device when coupled thereto; andabattery pack, configured to be removably inserted into the electronicdevice, comprising: at least two electrochemical cells arranged in aseries configuration to deliver power to the electronic device when theelectronic device is not coupled to the power unit, the seriesconfiguration forming at least one intermediate node; chargingcircuitry, coupled to the at least two electrochemical cells and the atleast one intermediate node, that independently controls charging ofeach of the at least two electrochemical cells from power provided bythe power unit when the electronic device is coupled to the power unit;and the charging circuitry including a voltage equalizer.
 8. The batterysystem of claim 7 wherein the charging circuitry attempts to preventovercharging of any of the at least two electrochemical cells whileattempting to fully charge all of the at least two electrochemicalcells.
 9. The battery system of claim 7 wherein the at least twoelectrochemical cells comprise lithium type cells.
 10. The batterysystem of claim 7 wherein the charging circuitry includes a currentregulator coupled in series with the at least two electrochemical cells.11. The battery system of claim 7 wherein the charging circuitrysimultaneously charges the at least two electrochemical cells while inthe series configuration via access to the at least one node.
 12. Thebattery system of claim 7 wherein the battery pack and power unitattempt to provide continuous power delivery to the electronic devicewhile either the battery pack or the power unit couples or uncouplesfrom the electronic device.
 13. A battery system for an electronicdevice comprising:a power unit, external to the electronic device, thatdelivers power to the electronic device when coupled thereto; andabattery pack, configured to be removably inserted into the electronicdevice, comprising: at least two electrochemical cells arranged in aseries configuration to deliver power to the electronic device when theelectronic device is not coupled to the power unit; charging circuitry,coupled to the at least two electrochemical cells, that independentlycontrols charging of each of the at least two electrochemical cells frompower provided by the power unit when the electronic device is coupledto the power unit; and the charging circuitry including a currentregulator coupled in series with the at least two electrochemical cells.14. The battery system of claim 13 wherein the charging circuitryattempts to prevent overcharging of any of the at least twoelectrochemical cells while attempting to fully charge all of the atleast two electrochemical cells.
 15. The battery system of claim 12wherein the at least two electrochemical cells comprise lithium typecells.
 16. The battery system of claim 12, further comprising:the seriesconfiguration of the at least two electrochemical cells forming at leastone intermediate node; and the charging circuitry including a voltageequalizer coupled to the at least two electrochemical cells and the atleast one intermediate node.
 17. The battery system of claim 12 whereinthe charging circuitry simultaneously charges the at least twoelectrochemical cells while in the series configuration via access tothe at least one node.
 18. The battery system of claim 12 wherein thebattery pack and power unit attempt to provide continuous power deliveryto the electronic device while either the battery pack or the power unitcouples or uncouples from the electronic device.